The present invention relates generally to dc power supplies, and more particularly to a method and apparatus for accurately controlling an off-line dc power supply while ensuring isolation of the primary and secondary sides of the power supply circuit.
An "off-line" power supply is one which is directly operated from an alternating current (ac) line source with no isolation between the ac source and the power supply. Accordingly, for safety reasons and to obtain an Underwriter's Laboratory (UL) seal of approval, isolation between the ac line source and the dc output(s) of the power supply must be failsafe.
To this end, off-line power supplies in the past have been regulated either by controlling a primary side output, or a secondary side output by means of an isolated feedback of the secondary side output through a transformer or an optocoupler. Unfortunately, control of a primary side output results in poor regulation of the power supply, and transformers and optocouplers are relatively expensive, and therefore preferably should be eliminated from the power supply if possible. Further, available integrated circuits which can control the power supply in response to an adequately isolated secondary output signal also add to the cost of the power supply and consume in excess of one half watt of power.
Accordingly, there is a need for an inexpensive primary/secondary link to permit satisfactory regulation of off-line operated power supplies and also to ensure isolation of the primary side of the power supply circuit from the secondary side. It would be further advantageous if a control circuit for operation with such a primary/secondary link consumed very little power and, in particular, well below that consumed by commercially available integrated circuit regulators.